Complex interactions of the pituitary, hypothalamus, and pineal involving MIF-1 (Pro-Leu-Gly-NH<sub>2</sub>) were shown many years ago. One of the largest changes consisted of increased amounts of an MIF-1-like material in the plasma of hypophysectomized rats as measured by the skin lightening of darkened frogs. The present study used radioimmunoassay to measure immunoreactive Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH<sub>2</sub>) in intact, hypophysectomized, pinealectomized, and superior cervical ganglionectomized (SCG) rats. Samples were collected every 4 h for 24 h. Plasma concentrations of Tyr-MIF-1-like immunoreactivity were higher in hypophysectomized rats and lower in pinealectomized rats than in the intact or SCG groups, which were not reliably different from each other. In most groups, the highest plasma concentrations appeared to occur about 04.00 h, a time at which brain concentrations of Tyr-MIF-1-like immunoreactivity were low. However, no diurnal rhythm in plasma was seen in the pinealectomized rats. At none of the times did brain concentrations of immunoreactive peptide differ among the four groups. By high-performance liquid chromatography of plasma, the main peak of immunoreactivity was found to elute at the same position as Tyr-MIF-1 supporting the natural occurrence of this tetrapeptide. Thus, this study demonstrates that the concentration of immunoreactive Tyr-MIF-1 in plasma is increased by hypophysectomy.